Detached macrophytes (seagrass and macroalgae) are transported from more offshore areas and accumulate in substantial volumes in surf zones, where they are commonly called wrack. Fishes were sampled using seine nets in 4 volume categories of detached macrophytes (bare sand, low, medium and high volumes) in the surf zone at 2 sandy beaches in southwestern Australia to determine how increasing volumes of surf-zone wrack influences fish community and size composition. Species composition and densities of fish, which were dominated by juveniles, differed between areas where wrack was present or absent, and also among volumes of wrack in the surf zone. Total fish abundance and biomass increased as the volume of wrack increased. Cnidoglanis macrocephalus and Pelsartia humeralis were the dominant species and were most abundant in medium and high wrack volumes. Fish gut contents were analysed for C. macrocephalus and P. humeralis, and verified that Allorchestes compressa is a major prey item for juveniles of these species. A series of habitat preference trials conducted in outdoor aquaria tested whether juvenile C. macrocephalus and P. humeralis showed a preference for different types of detached macrophytes as a habitat, i.e. seagrass, brown algae, or a mixture of both macrophyte types. Non-parametric goodness-of-fit binomial tests for differences in the number of fish between each habitat type showed no clear pattern in habitat preference for either species of fish. Field and laboratory results suggest that the amount, rather than type, of detached macrophytes is more important in providing a habitat for juvenile C. macrocephalus and P. humeralis.

In several parts of the world, detached seagrass and macroalgae accumulate in the surf zone where in situ primary production can be low. This allochthonous resource is therefore likely to be important to consumers, but the various components of the resource likely play different roles in the food webs. We traced sources of production for the abundant amphipod Allorchestes compressa and 2 key predatory fishes in the surf zone of southwestern Australia using multiple stable isotopes (δ13C, δ15N and δ34S) and fatty acids. Seagrasses had higher δ13C than macroalgae, while δ13C of red and brown macroalgae were similar. Mixing models based on δ13C were ambiguous and indicated that brown algae and seagrasses were both feasible carbon sources for A. compressa. δ15N varied little between seagrasses and macroalgae, while δ15N of amphipods was higher and δ15N of fish higher again. δ34S was not useful in distinguishing between macrophytes or trophic levels. The fatty acid composition of brown and red algae and seagrasses were distinct. The composition of essential fatty acids (i.e. fatty acids that the consumer must obtain through its diet) of A. compressa was most similar to that of brown algae. The combination of stable isotope and fatty acid analyses indicates that, although brown algae comprises a lower proportion of wrack biomass than seagrass (17 to 28% vs. 52 to 58%), brown algae, particularly the kelp Ecklonia radiata, contributes disproportionately to the surf-zone food web in southwestern Australia

Allorchestescompressa is the dominant macroinvertebrate species in wrack accumulations on surf zones of south-western Australia. These amphipods were provided with a choice of macrophyte material representing brown and red algae and seagrass in a series of preference experiments in the laboratory and field. Feeding experiments showed that A. compressa exhibited a strong preference for particular types of macrophytes (P \u3c 0.01). Amphipods primarily consumed brown algae, with 69–98% of the biomass of Ecklonia radiata and 64% of the biomass of Sargassum sp. lost over the experiments. This study has shown that the amphipod A. compressa exhibits a clear preference for brown algae over red algae and seagrass as food. In terms of habitat preference, tank experiments using a series of pair-wise comparisons showed that, in the absence of fish predators, A. compressa selected seagrass as its preferred habitat over the other types of wrack (P \u3c 0.001). When satiated or starved, between 68 and 83 and 79 and 98% of amphipods were found in Amphibolis and Posidonia, respectively. In contrast, field-cage experiments revealed that A. compressa preferred either mixed wrack, brown algae or red algae over seagrass as a habitat (P \u3c 0.01). The contrasts between results from the laboratory and field suggest that other factors such as the presence of predators, water flow and light could influence habitat choice in the surf zone. This study shows that allochthonous material transported to surf zones from other habitats therefore play different roles in driving secondary production in this shoreline habitat

No abstract